The invention relates to a subscriber for a communication system, particularly an Ethernet and/or real-time Ethernet communication system, for transmitting and receiving data telegrams. The invention further relates to a communication system and a communication method.
A synchronous, clocked communication system with equidistance properties is a system comprising at least two subscribers, which are connected to one another via a data network so as to mutually exchange data or mutually transmit data. Therein, data interchange occurs cyclically in equidistant communication cycles. The communication cycles are predefined by the communication clock of the system.
Equidistant deterministic cyclic data interchange in communication systems is based on a common clock or time base for all the components involved in the communication. The clock or time base is transmitted by a distinguished component (clock generator) to the other components. In the case of an isochronal real-time Ethernet, the clock or the time base is prescribed by a synchronization master by means of transmitting synchronization telegrams.
Exemplary embodiments of subscribers include central automation devices; programming, configuration or control devices; peripheral devices such as input/output assemblies, drivers, actuators, or sensors; stored program controls (SPCs) or other control units; computers; or machines, which interchange electronic data with other machines, particularly machines which process data from other machines. The subscribers are also called network nodes or nodes.
Control units are, for example, regulator or control units of all types, switches and/or switch controllers. Exemplary embodiments of data networks are, for example, bus systems, such as field bus, process data highway, Ethernet, Industrial Ethernet, FireWire or PC-internal bus systems (PCIs), etc. In particular, an exemplary embodiment of a data network is an isochronal real-time Ethernet.
Data networks allow communication between a plurality of subscribers by networking, i.e., by connecting the individual subscribers to one another. Therein, the term “communication” means the transmission of data between the subscribers. The data to be transmitted are sent in the form of data telegrams, i.e., the data are packed together to form a plurality of packets and are sent in this form to the appropriate receiver via the data network. Therefore, these packets are also referred to as data packets. In this context, the term “transmission of data” is used synonymously with the aforementioned transmission of data telegrams or data packets.
In distributed automation systems, for example in the area of drive technology, certain data need to arrive at appropriate subscribers and be processed by the receivers at certain times. This is called real-time-critical data and data traffic, since any delayed arrival of the data at the intended location leads to unwanted results at the subscriber. This is in contrast to data communication that has no real-time criticality, for example Internet-based or intranet-based data communication. According to IEC 61491, EN61491 SERCOS interface—Brief Technical Description, successful real-time-critical data traffic of the above-mentioned type can be ensured in distributed automation systems.
Today, automation components (e.g., controls, drives, . . . ) generally have an interface to a cyclically clocked communication system. An execution level of the automation component (fast-cycle) (e.g., positional regulation in a control, torque regulation in a drive) is synchronized to the communication cycle. Thereby, the communication clock is determined. Likewise, other, low-performance algorithms (slow-cycle) (e.g. temperature regulations) for the automation component can communicate with other components (e.g. binary switches for fans, pumps, . . . ) using this communication clock only, even though a slower cycle would be sufficient. Using just one communication clock for transmitting all the information in the system results in high demands on the transmission link's bandwidth.
Process control and monitoring in automated production and, particularly, in the field of digital drive technologies requires very fast and reliable communication systems with predictable reaction times.
German patent application DE 100 58 524.8 discloses a system and a method for transmitting data via switchable data networks, particularly the Ethernet, which permits hybrid or mixed operation of real-time-critical and non-real-time-critical data communication, in particular Internet-based and intranet-based data communication. This allows both real-time-critical (RT; real-time) and non-real-time-critical (NRT; non-real-time) communication in a switchable data network of, e.g., an distributed automation system through cyclic operation. Therein, the data network includes subscribers and coupling units.
A “transmission cycle” respectively has, for all the subscribers and coupling units in the switchable data network, at least one area for transmitting real-time-critical data and at least one area for transmitting non-real-time-critical data. Thereby, the real-time-critical communication is separated from the non-real-time-critical communication. Since all the subscribers and coupling units are always synchronized to a common time base, the respective areas for transmitting data for all the subscribers and coupling units each occur at the same time. In other words, the real-time-critical communication is independent of that of the non-real-time-critical communication with respect to time, and the real-time-critical communication is therefore not affected by the non-real-time-critical communication. The real-time-critical communication is planned in advance. The data telegrams are supplied at the original transmitter and forwarded by the involved coupling units in time-based fashion. Due to the buffer-storing in the respective coupling units, spontaneous, Internet-compatible, non-real-time-critical communication, which occurs at any time, is shifted to the transmission cycle's transmission area that is provided for non-real-time-critical communication. In addition, this communication is transmitted in that area only.
The above-cited application illustrates, by way of example, a basic structure for a transmission cycle that is split into two areas. A transmission cycle is split into a first area, which is provided for transmitting real-time-critical data, and a second area, which is provided for transmitting non-real-time-critical data. The length of the illustrated transmission cycle symbolizes its duration, which is, advantageously, between a few microseconds and a few seconds, for example, depending on the application.
The duration or time period of a transmission cycle is variable. However, the duration of the transmission cycle is defined, for example by a control computer, at least once before the point in time of transmitting the data. Moreover, the duration of the transmission cycle has the same respective length for all subscribers and coupling units in the switchable data network. The time period of a transmission cycle and/or the time period of the first area, which is provided for transmitting real-time-critical data, can be altered at any time. For example, such alteration can occur at previously planned, fixed times and/or after a planned number of transmission cycles, advantageously before the start of a transmission cycle, in that the control computer swtiches over to, e.g., other planned, real-time-critical transmission cycles.
In addition, if necessary, the control computer can re-plan the real-time communication at any time during the ongoing operation of an automation system. Thereby, the time period of a transmission cycle can be changed too. The absolute time period of a transmission cycle is a measure of the time component and/or the bandwidth of the non-real-time-critical communication during a transmission cycle, i.e., the time that is available for non-real-time-critical communication. Thus, with a time period for real-time-critical communication of 350 μs and a transmission cycle of 500 μs, for example, the non-real-time-critical communication has a bandwidth of 30%. In the case of 10 ms, the bandwidth of the non-real-time-critical communication is 97%.
In the first area, which is provided for transmitting real-time-critical data, a certain time period for transmitting data telegrams that organize the data transmission is reserved, prior to transmitting the actual real-time-critical data telegrams. By way of example, the data telegrams for organizing the data transmission contain data for time synchronization of the subscribers and coupling units in the data network and/or data for topology recognition in the network.
After these data telegrams have been transmitted, the real-time-critical data telegrams are transmitted. Since, as a result of the cyclic operation, the real-time communication can be planned in advance, the transmission times and/or the times for forwarding the real-time-critical data telegrams are known, prior to the start of data transmission, for all real-time-critical data telegrams to be transmitted. In other words, the time period of the area for transmitting non-real-time-critical data is automatically defined by the time period of the area for transmitting real-time-critical data.
It is an advantage of this arrangement that only the respective necessary transmission time for the real-time-critical data traffic is used. After the real-time-critical data traffic has ended, the remaining time is automatically available for non-real-time-critical communication, for example for unplannable Internet communication and/or other, non-real-time-critical applications. It is a particular advantage that the time period of the area for transmitting real-time-critical data is respectively determined by the data that are to be transmitted on a connection-specific basis. In other words, the time period of the two areas is determined, for each individual data link, by the required data volume for the real-time-critical data to be transmitted. Thereby, the time split for the two areas can be different for each individual data link of each transmission cycle.
Only the respective, necessary transmission time for the real-time-critical data traffic is used. The remaining time of a transmission cycle is automatically available for non-real-time-critical communication, for example for unplannable Internet communication and other, non-real-time-critical applications for all subscribers in the switchable data network.
Since the real-time communication is planned in advance such that the arrival of the real-time-critical data telegrams in the respective coupling units is planned so that the real-time-critical data telegrams under consideration arrive at the respective coupling units no later than the forwarding time, the real-time-critical data telegrams can be transmitted or forwarded without any interim time window. Thereby, due to the tightly-packed transmission and/or forwarding, the available time period is used in the best possible way. Naturally, if necessary, it is also possible to incorporate transmission breaks between the transmission of the individual data telegrams.
As a representation for any network, the basic manner of operation in a switched network is explained below, by way of example, with reference to two subscribers (e.g., a drive and a control computer) that have respective integrated coupling units, and with reference to a further subscriber without a coupling unit, which are connected to one another by data links.
The coupling units each have local memories, which are connected to the subscribers via internal interfaces. The subscribers use the interfaces to interchange data with the respective coupling units. Within the coupling units, the local memories are connected to the control circuits via the data links. The control circuits receive data and/or forward data via the internal data links from and/or to the local memories or via one or more of the external ports. By applying the method of time synchronization, the coupling units always have a common synchronous time base. If a subscriber has real-time-critical data, then these real-time-critical data are picked up, via the respective interface and the local memory, from the appropriate control circuit at the preplanned time during the area for the real-time-critical communication. From there, the real-time-critical data are transmitted to the next connected coupling unit via the respectively provided external port.
If another subscriber transmits non-real-time-critical data, for example for an Internet request, at the same time, i.e., during the real-time-critical communication, then these non-real-time-critical data are received by the control circuit via the external port and are forwarded via an internal communication to the local memory, where they are buffer-stored. From there, they are picked up again only in the area for the non-real-time-critical communication, and then forwarded to the receiver. In other words, the non-real-time-critical data are shifted to the second area of the transmission cycle, which is reserved for spontaneous, non-real-time-critical communication. Thereby, interference with the real-time communication is prevented.
If not all the buffer-stored, non-real-time-critical, data can be transmitted during that area of a transmission cycle that is provided for transmitting the non-real-time-critical data, then the non-real-time-critical data are buffer-stored in the local memory of the respective coupling unit, until they can be transmitted during an area of a later transmission cycle that is provided for transmitting the non-real-time-critical data. This reliably prevents interference with the real-time communication.
The real-time-critical data telegrams which arrive, via respective data links and via the external ports, at the associated coupling unit's control circuit, are forwarded directly via the respective external ports. This is possible because the real-time communication is planned in advance. Hence, for all the real-time-critical data telegrams to be transmitted, the transmission and reception times; all of the respective coupling units involved; all of the forwarding times; and all the receivers of the real-time-critical data telegrams are known.
The advance planning of real-time communication also ensures that no data collisions occur on the data links. Likewise, the forwarding times for all the real-time-critical data packets from the respectively involved coupling units are planned in advance and are, thus, clearly defined. Therefore, the arrival of the real-time-critical data telegrams is planned such that the real-time-critical data telegrams under consideration arrive in the respective coupling unit's control circuit no later than the forwarding time. As a result, the problem of time ambiguities, which become noticeable particularly in the case of long transmission chains, is eliminated. Consequently, as stated above, simultaneous operation of real-time-critical and non-real-time-critical communication in the same switchable data network, and any connection of additional subscribers to the switchable data network are possible without having a disruptive effect on the real-time communication itself.
The method described in German patent application DE 100 58 524.8 allows to set up Ethernet-based communication networks, in particular isochronal Ethernet-based communication networks. The subscribers of these networks interchange data records with very high frequency and make them available to the user. Therein, the hardware support permits the user interface to have a throughput that can keep up with the maximum possible telegram volume of the connected links. With four connected 100 Mbit full duplex links and frames of 64 bytes in length, for example, the telegram volume is approximately 1 000 000 telegrams/s. In contrast to this, the throughput of software/communication-stack-based user interfaces for spontaneous communication is at least two orders of magnitude smaller.
However, these high throughput rates are available only for isochronal cyclic communication, for which, at the reception end too, preplanned reception times for telegrams are strictly observed. This means that the telegram transmission requires a network that can perform the method of time-based connection described in the German patent application DE 100 58 524.8. However, a high-performance user interface, which is also able to cooperate with existing networks that have an address-based interconnection, is highly desirable.